The present invention relates to a method and an apparatus for producing ammonia (NH3) and introducing the produced ammonia (NH3) into an exhaust gas stream as a reduction means for selectively catalytically reducing nitrogen oxides contained in the exhaust gas stream, which is an exhaust stream generated by the combustion process of a motor, a gas turbine, or a burner.
Ammonia (NH3) serves in the above-noted applications of interest as a reduction means which is introduced, as viewed relative to the exhaust gas flow, upstream of a special SCR catalyzer or catalytic converter, or upstream of an integrated group of SCR catalyzer modules, disposed for exhaust gas flow therethrough in a parallel manner, built into a muffler in the exhaust gas conduits of internal combustion engines, gas turbines, or burners or introduced into SCR catalyzer systems in which a reduction of the nitrogen oxides contained in the exhaust gas is effected. The term “SCR” means selective catalytic reduction.
Various conventional methods are known for the production of ammonia (NH3) which are discussed in detail below.
EP 0487886 A2 proposes a method for a quantitative disassociation, via hydrolysis, of an aqueous urea solution into ammonia (NH3) and carbon dioxide (CO2) in a temperature range of 160° C. to 550° C., in which, in the resultant product, undesired reaction products such as isocyanic acid (HCNO) and carbon monoxide (CO) can be avoided. In this known method, the urea solution is initially sprayed via a spray nozzle onto a vaporizer/catalyzer disposed in the exhaust gas stream. The gas-forming products which are produced thereby are subsequently treated in a hydrolysis catalyzer before they are conducted through an SCR catalyzer for the reduction of nitrogen oxides therein.
EP 0555746 A2 discloses a method which enhances the method disclosed in EP 0487886 A2. Namely, the method disclosed in EP 0555746 A2 is enhanced in that the vaporization element thereof is configured such that the urea solution is handled in a homogeneous manner and distributed so as to ensure contact of the drops with the channel walls. The deposition of deposits onto the catalyzers and urea slip is avoided due to a homogeneous distribution. The urea dosing should only be active at an exhaust gas temperature above 160° C., since undershooting this temperature forms undesired byproducts.
A disadvantage of the aqueous urea solution is that its production is, by reason of the necessity of de-ionized water, comparatively expensive. De-ionized water is required to continuously protect the catalyzer surfaces from the deposition thereon of lime. Due to the high water portion of the urea solution (32.5% solution), the disadvantages of a significantly higher weight and larger facility volume must be dealt with in connection with such a solution, in comparison to dry urea or a mixture of urea with a reduced water portion.
A further disadvantage of the aqueous urea solution obtains with respect to its fitness for winter conditions. The freezing point of a 32.5% urea solution is −11° C. In the event of very low temperatures, the supply container and the dosing system must be heated. Moreover, in order to achieve the vaporization of the urea solution, 80% of the energy dedicated to the process must be allocated for such vaporization. This leads to a lowering of the exhaust gas temperature and, consequently, in the event of low temperatures under certain conditions, to a reduced conversion of nitrogen oxides.
EP 0615777 B1 proposes, in lieu of an aqueous urea solution, dry urea as the reduction means for SCR technology, in the form of prills with a diameter up to 5 millimeters or micro-prills with a diameter between 10 and 1,000 microns. The -prills are injected into the exhaust gas conduit by means of a dosing injection apparatus with the assistance of pressurized air. The danger exists, in connection with the deployment of dry urea, that the micro-prills are baked to one another or, respectively, adhere to one another. The flow capability of the micro-prills must be maintained to avoid deterioration of the dose injection. It has been suggested to deploy packing of micro-prills sealed against air or the dusting or powder application of micro-prills with organic nitrogen compounds.
EP 0878608 A2 discloses a melt product of pure urea as a reduction means for selective catalytic or non-catalytic nitrogen oxide reduction. The dry urea is initially present in the form of powder or in crystalline form and stored in a supply container. A predetermined amount of urea is fed to the heat transfer element via a feed worm as a function of the load of the motor and its rate of rotation. The heat transfer element, over which flows an exhaust gas stream, exploits the exhaust gas heat content to effect fluidizing of the solid urea. This process takes place at temperatures between 133° C. (the melting temperature of urea) and 250° C. It is desired to run the process at temperatures only up to 160° C. in order to avoid undesired byproducts such as, for example, cyanuric acid. The resulting melt product comprises the actual reduction means, ammonia (NH3), and, upon occasion, isocyanic acid (HCNO). The melt product is injected via a high-pressure spray pump through one or several injection nozzles into the exhaust gas. The location of the injection is, in SCR processes, immediately before the catalyst and, in connection with non-catalytic processes, immediately before the burner space.
U.S. Pat. No. 5,809,775 discloses converting by heating a solid suitable for NOX reduction in a gas generator into an ammonia (NH3)-containing gas. In this connection, the preferred NOx reduction means, urea, is heated to at least 300° C. via treatment by pyrolysis, wherein other reduction means, such as, for example, ammonium carbamate, need only be heated to 40° C. The ammonia (NH3)-containing gas is subsequently introduced into contact with the exhaust gas via an injection nozzle. The gas/exhaust gas mixture is then conducted through an SCR catalyzer. U.S. Pat. No. 5,809,775 discloses a comprehensive system which, building on the possibilities of providing a supply of the solid reduction means in a container with an outlet, offers further possibilities to heat the reduction means in a container, to produce the ammonia (NH3)-containing gas, and, subsequently, to conduct the gas to the exhaust gas and to conduct the gas/exhaust gas mixture through the SCR catalyzer. In one embodiment, the container for the supply and for the pyrolysis process are placed under pressure.
U.S. Pat. No. 5,968,464 discloses a method and an apparatus for NOX reduction in which an aqueous urea solution is sprayed onto a heated surface of a pyrolysis chamber, which comprises an outlet opening for the discharge therethrough of the gas-forming products of the urea pyrolysis and the water vaporization. In this connection, the solution remains in contact with the heated top surface of the pyrolysis chamber until the urea has been completely pyrolically treated and the water has been vaporized. The gas-forming reaction products are conducted to the exhaust gas system for selective catalytic reduction thereof at a suitable temperature.
WO 98/42623 discloses a method and an apparatus for the reduction of nitrogen oxide. In a preferred version, an aqueous urea solution with a concentration of about 10% to 50% by mass of urea is sprayed into a pyrolysis chamber. This reactor has its typical operational temperature at between 125° C. and 180° C. and an operational gas pressure of about 20 psig to 200 psig. The residence time of the reactive fluid in the reactor is selected such that the gas-forming reaction product, which is comprised of ammonia (NH3), carbon dioxide (CO2), and water vapor, is removed under controlled pressure and mass flow conditions from the chamber and, consequently, is maintained at at least 60° C. Viewing the entire range of possibilities, a urea concentration of 1% to 77% is used, the temperatures are at about 110° C. to 300° C., and the operational gas pressures are at 30 psig to 500 psig. The temperature and gas pressure are controlled via the introduction of heat into the reactor.
JP 63198713 A discloses a method in which nitrogen oxide is removed from the exhaust gas of diesel motors. In accordance with this known method, the exhaust gas is mixed together with ammonia (NH3), which is formed by the reaction of urea and water. The exhaust gas is brought into contact with a TiO2—catalyzer.
DE 19903439 A1 discloses a method for the dosing of reduction means in which the basic amount of reduction means to be dosed is determined by the magnitude of the load and at least one further operational parameter such as, for example, the rotational speed. Moreover, the dosing amount is adjustable as a function of at least one further operational parameter, such as, for example, the exhaust gas temperature.
DE 19901915 A1 suggests a dosing strategy for exploiting the reduction means storage capability of an SCR catalyzer, in which the reduction means is dosed up to a limit which results in an over stoichiometric dosage of the reduction means. Thereafter, the dosing is terminated or is further continued but at an under-stoichiometric rate. The advantage of the process is found in a higher average nitric oxide conversion, especially during varying operational conditions.
DE 19855385 A1 discloses a reduction means dosing and vaporization apparatus in which the reduction means is introduced via a (heated) nozzle located on the exhaust gas conduit. Due to the heating of the nozzle to 300° C. to 360° C., the reduction means is substantially vaporized.
In order to promote the conversion of urea into ammonia (NH3), it is proposed, in DE 19913462 A1, to deploy a reactor into which a portion of the exhaust gas stream—a partial exhaust gas stream—is conducted. A reduced flow speed is present in the partial exhaust gas stream as compared to the full exhaust gas stream and, consequently, the residence time of the urea introduced into the reactor is correspondingly greater and more time is available for chemical conversion.
DE 19922959 A1 discloses an exhaust gas cleaning apparatus in which the reduction means is introduced in vaporized form into a nitrogen oxide reduction catalyst. The vaporizer comprises, as a heat source, a baffle, which can be heated, onto which the fluidized reduction means is disposed under pressure. In order to convert the urea into ammonia (NH3), the baffle surface can be coated with a catalytic material. The introduction by dosing of the reduction means can follow thereafter by introduction of the reduction means into the principal exhaust gas stream or in a partial exhaust gas stream branched from the principal exhaust gas stream.
DE 19949296 A1 discloses a heating apparatus for a reduction means pressure body, which is connected to a gas collection space. The gas-forming reduction means which is produced thereby is introduced via a spray valve into a reaction chamber disposed in the exhaust gas stream for thermal conversion of the reduction means in the reaction chamber. The reaction chamber is communicated into the exhaust gas conduit.